Generally, graphene is a semimetallic material where carbon atoms form an arrangement connected in a hexagonal shape by two-dimensional sp2 bonding while having a thickness corresponding to a carbon atom layer. Recently, it has been reported that the properties of a graphene sheet having one carbon atomic layer were evaluated, and as a result, the graphene sheet may show very excellent electrical conductivity of electron mobility of about 50,000 cm2/Vs or more.
Further, graphene has the properties of structural and chemical stability and excellent thermal conductivity. In addition, graphene is consisting of only carbon which is a relatively light element, and thus, easy to be processed in one-dimensional or two-dimensional nanopatterns. Most of all, the graphene sheet is inexpensive materials and has excellent price competitiveness, as compared with existing nanomaterials.
Due to such electrical, structural, chemical and economical properties, graphene is expected to replace a silicon-based semiconductor technology and a transparent electrode in the future, and especially, is possible to be applied to a flexible electronic device field due to excellent mechanical properties.
Due to the numerous advantages and excellent properties of the graphene, various methods capable of more effective mass production of the graphene from carbon-based materials such as graphite, have been suggested or studied. Particularly, a method capable of preparing a graphene sheet or flake with less defect generation, and having a smaller thickness and a large area by a more simplified process has been studied in various ways, so that excellent properties of the graphene are more dramatically expressed. The existing methods of preparing graphene as such include the following:
First, a method wherein a graphene sheet is exfoliated from graphite by a physical method such as using a tape, is known. However, such method is not suitable for mass production, and has a very low exfoliation yield.
Further, another method wherein graphite is exfoliated by a chemical method such as oxidation, or acid, base, metal, and the like are inserted between the graphite carbon layers to obtain graphene or an oxide thereof which is exfoliated from an intercalation compound, is known. However, the former method may generate a number of defects on finally prepared graphene, in the course of obtaining graphene by proceeding with exfoliating by oxidation of graphite, and reducing a graphene oxide obtained therefrom again to obtain graphene. This may adversely affect the properties of finally prepared graphene. Further, the latter method also requires further processes such as using and treating the intercalation compound, and thus, the overall process is complicated, the yield is insufficient, and the economics of the process may be poor. Moreover, it is not easy to obtain a graphene sheet or flake having a large area in such a method.
Due to the problems of those methods, recently, a method of preparing graphene by exfoliating carbon layers contained in graphite by a milling method using ultrasonic irradiation, a ball mill or the like, in a state of dispersing graphite and the like in liquid, is applied the most. However, such methods also had problems of being difficult to obtain graphene having sufficiently small thickness, generating a number of defects on graphene in an exfoliating process, having insufficient exfoliating yield and a mass production property, or the like.
In addition, a method of preparing graphene by exfoliating the graphite and the like using a homogenizer such as a high speed homogenizer, has also been suggested. However, in the existing method as such, it was common that mainly the raw material such as graphite is oxidized, or subjected to high-temperature heat treatment and crushing, thereby forming a graphite worm or oxidized graphite, which is then exfoliated to prepare graphene. Nevertheless, a number of defects were generated on the raw material during the high-temperature heat treatment and crushing process, thereby greatly reducing the thermal, electrical or mechanical properties of the finally prepared graphene. Further, due to the need of progress of the high-temperature heat treatment and crushing process, and the like, the overall process may be complicated, and it became difficult to prepare graphene having sufficiently large area.
Moreover, in case of exfoliating the oxidized graphite, the exfoliated product therefrom was obtained as oxidized graphene containing defects and oxygen, and thus, it had poor electrical conductivity, as compared with general graphene. In order to solve the problems, a reduction process of oxidized graphene and the like were additionally required for obtaining graphene. As a result, the overall process became more complicated, and even after reduction, the physical properties of the graphene were not completely restored to those before oxidation.
Because of these, a preparation method of graphene, capable of preparing graphene having a smaller thickness and a large area, and with reduced defect generation, thereby maintaining excellent properties, by a simplified process, has been continuously demanded.